Factor the following expression: $4x^2 - 81$
Answer: The expression is of the form ${a^2} - {b^2}$ , which is a difference of two squares so we can factor it as $({a} + {b}) ({a} - {b})$ What are the values of $a$ and $b$ $ a = \sqrt{4x^2} = 2x$ $ b = \sqrt{81} = 9$ Use the values we found for $a$ and $b$ to complete the factored expression, $({a} + {b}) ({a} - {b})$ So we can factor the expression as: $({2x} + {9}) ({2x} - {9})$